1. Field of the Invention
The present invention relates to piezoelectric acoustic components, such as a piezoelectric buzzer and a piezoelectric earphone.
2. Description of Related Art
Generally, in electronic devices, household-electric-appliances products, portable telephones, or a piezoelectric earphone and other such products, a piezoelectric acoustic component has been widely used as a piezoelectric buzzer or a piezoelectric earphone which generates a warning sound and a sound of operation.
This kind of piezoelectric acoustic component is disclosed in Japanese unexamined patent publication No. 7-107593 and Japanese unexamined patent publication No. 7-203590, for example. The piezoelectric acoustic component has a unimorph type diaphragm including a circular piezoelectric plate and a circular metal plate disposed on an electrode of the circular piezoelectric plate. The circular metal plate of the diaphragm is supported in a circular case at the circumference portion of the circular metal plate, and the opening of the circular case is sealed by a cover.
However, when such diaphragm having such a circular shape is used, problems such as a very low production efficiency and low acoustic conversion efficiency occur. Further, it is difficult to reduce the size of the piezoelectric acoustic component.
The problem of the very low production efficiency will be explained hereinafter.
In a manufacturing process of a piezoelectric acoustic component including the diaphragm having a circular shape, a green sheet 40 is punched by a punching die 41 to produce circular piezoelectric plates 42 as shown in FIG. 1. Next, a circular metal plate 43 is disposed at a electrode on one side of each of the circular piezoelectric plates 42. Then, a diaphragm 44 is obtained by applying a high voltage DC electric field between the electrodes on both sides of the circular piezoelectric plate 42 and polarizing the circular piezoelectric plates 42. The diaphragm 44 is stored in a case 45, and lead wires 46 and 47 respectively connected to the other surface electrode and the metal plate 43 of the piezoelectric board 42 are arranged to extend out of the case 45.
However, when the circular piezoelectric plate 42 is punched from a green sheet 40, as mentioned above, punching sediment increases and the yield of material is very low. Moreover, the process efficiency is very low because the individual processes of electrode formation, polarization, and other steps must be done after the punching process. Furthermore, in order to determine the required size for the design of each element, the punching die 41 of a green sheet must be produced according to the element size. Therefore, the production efficiency is very low and the cost of manufacturing is very high.
The problem with the low acoustic conversion efficiency will be explained hereinafter.
As shown in FIG. 2A, since the diaphragm 44 is supported by a case 45 at the circumference portion thereof, the maximum displacing point P is only at the center portion of the diaphragm 44. Therefore, the displacement volume is small and the acoustic conversion efficiency is low. That is, there is a disadvantage that sound pressure per input energy is comparatively low.
Furthermore, the frequency is high because the circumference portion of the diaphragm is constrained. When producing the piezoelectric diaphragm having a low frequency, there is a disadvantage that the radius size is increased and becomes very large.
To overcome the above described problems, preferred embodiments of the present invention provide a piezoelectric acoustic component which achieves a high production efficiency, excellent acoustic conversion efficiency and a very small size.
A preferred embodiment of the present invention provides a piezoelectric acoustic component which includes a diaphragm having a substantially rectangular piezoelectric plate including front and back surfaces, an electrode disposed on the front surface of the piezoelectric plate, a substantially rectangular metal plate bonded to the back surface of the rectangular piezoelectric plate directly or via an electrode disposed on the back surface of the rectangular piezoelectric plate, an insulating cap including an upper wall, four side walls extending from the upper wall, a pair of support members arranged to support the diaphragm at the inside of the side walls, a plate shaped substrate having a first electrode section and a second electrode section, the diaphragm being disposed in the insulating cap, two of four side edges of the diaphragm disposed opposite to each other being fixed to the pair of support members, a gap between the other two of the four side edges of the diaphragm and the cap being sealed by elastic sealing material, an acoustic space being provided between the diaphragm and the upper wall of the insulating cap, an opening edge of the four side walls of the insulating cap being bonded to the substrate, the metal plate being electrically connected to the first electrode section, and the electrode disposed on the front surface of the substantially rectangular piezoelectric plate being electrically connected to the second electrode section.
Since the piezoelectric plate has a substantially rectangular shape, even if the piezoelectric plate is produced by punching a green sheet, the generation of punching residue of the green sheet is minimized, and thereby the material efficiency is much improved.
Further, since the process of electrode formation, polarization, and other steps are performed on the parent substrate rather than individual components, production efficiency is very good.
Furthermore, since the required size of the piezoelectric plate is achieved by cutting the parent substrate, it is not necessary to prepare a punching die for punching individual green sheets as is required in the prior art.
In other words, compared with the prior art, the number of steps for punching a green sheet and cutting a parent substrate is greatly reduced and the number of dies, jigs, and types of piezoelectric bodies are reduced, thereby increasing production efficiency and reducing the cost of production.
In preferred embodiments of the present invention, two of four side edges of the diaphragm which are opposite to each other are fixed to the pair of support members, and the gap between the other two of the four side edges of the diaphragm and the cap are sealed by elastic sealing material. When a predetermined frequency signal is input between the metal plate and the electrode disposed on the front surface of the piezoelectric plate, the piezoelectric plate expands, and the diaphragm is deformed in a longitudinal bending mode. In this case, the diaphragm is vibrated so as to generate two nodes at both ends thereof which are fixed to the cap via the support members. As shown in FIG. 2B, a maximum displacement point P exists along the centerline in the longitudinal direction of the diaphragm.
That is, the displacement volume is very large compared with a conventional disc-shaped diaphragm. Since this displacement volume is the energy for moving air, the acoustic conversion efficiency is greatly increased.
According to the above described piezoelectric acoustic component, the gap between the other two of the four side edges of the diaphragm and the cap is preferably sealed by elastic sealing material. Since the elastic sealing material has elasticity, the vibration of the diaphragm is not interfered with, and the sound pressure is not decreased.
Furthermore, since the diaphragm is fixed at the two side edges thereof and the portion between the two side edges is arranged to be displaced freely, the component achieves a lower frequency compared with a conventional disc-shaped diaphragm. Alternatively, the size of the component is reduced when the same frequency is required.
In preferred embodiments of the present invention, the adhesive agent is Preferably one which has a high Young""s modulus in the hardened condition, and constrains the edge part of the diaphragm strongly.
Moreover, the elastic sealing material preferably has a low Young""s modulus in hardened condition, and the restraint of the diaphragm is weak enough to permit the vibration of the diaphragm. Since there are some elastic sealing materials which have adhesive strength for bonding the diaphragm and the cap, it is possible to use such materials in place of the adhesive agent.
According to the above described piezoelectric acoustic component, the insulating cap which fixes the diaphragm is bonded to the plate shaped substrate, the metal plate is electrically connected to the first electrode section, and the electrode disposed on the front surface of the substantially rectangular piezoelectric plate is electrically connected to the second electrode section. As a result, the piezoelectric acoustic component is completed. In addition, the piezoelectric acoustic component can be constructed as a surface mounting type component by extending the first and second electrode sections to the back-side of the substrate.
Preferably, the piezoelectric plate is disposed on the metal plate in such a way that respective longer edge sides of the piezoelectric plate and the metal plate are aligned to each other and a respective one of the two shorter edge sides of the piezoelectric plate and the metal plate are aligned with each other so that the piezoelectric plate is disposed on the metal plate leaning towards the one shorter edge side of the metal plate at which the metal plate is supported by the support members of the insulating cap. An exposed area is provided around the other shorter edge side of the metal plate and the diaphragm is fixed to the support members of the insulating cap so that the metal plate opposes the upper wall of the insulating cap.
It is possible to fix the diaphragm to the support members of the insulating cap so that the piezoelectric plate opposes the upper wall of the insulating cap, but in this case it is difficult to connect the electrode disposed on the front surface of the piezoelectric plate to the second electrode section of the substrate. On the other hand, when the diaphragm is fixed to the support walls of the insulating cap so that the metal plate opposes the upper wall of the insulating cap, the electrode disposed on the front surface of the piezoelectric plate opposes the substrate. As a result, it is easy to connect the electrode disposed on the front surface of the piezoelectric plate to the second electrode section of the substrate. Further, since the exposed area of the metal plate is exposed around one edge side of the diaphragm, it is easy to connect the metal plate to the first electrode section of the substrate.
Preferably, the metal plate is connected to the first electrode section by electroconductive glue and the electrode disposed on the front surface of the rectangular piezoelectric plate is connected to the second electrode section by electroconductive glue. In this case, the process of bonding the insulating cap to the substrate, the process of electrically connecting the metal plate to the first electrode section and the process of electrically connecting the electrode disposed on the front surface of the piezoelectric plate to the second electrode section can be performed at the same time. Thus, the connecting process is very simple and much less complicated and time consuming than conventional processes.
Preferably, the elastic sealing material includes an insulating material. The elastic sealing material is preferably provided at all of the four side edges of the diaphragm. It is noted that since the metal plate and the electrode disposed on the front surface of the piezoelectric plate are located near each other, they are likely to be short-circuited when connecting the electrode disposed on the front surface of the piezoelectric plate and the second electrode section by the electroconductive glue. If the elastic sealing material is provided in the periphery of the metal plate beforehand, such short-circuit can be prevented. Further, by sealing all of the four side edges of the diaphragm, air leakage is prevented to thereby greatly improve the sound pressure characteristic.
For providing the elastic sealing material at all of the four side edges of the diaphragm, a method of attaching the diaphragm to the insulating cap only by the elastic sealing material (without using adhesive) and a method of providing the elastic sealing material over the two edge sides of the diaphragm which are fixed by adhesive can be used. The latter method is advantageous when it is impossible to prevent air leakage only by the adhesive.
Another preferred embodiment of the present invention provides a piezoelectric acoustic component which preferably includes a diaphragm having a substantially rectangular piezoelectric plate including front and back surfaces, an electrode disposed on the front surface, a substantially rectangular metal plate bonded to the back surface of the rectangular piezoelectric plate directly or via an electrode disposed on the back surface of the substantially rectangular piezoelectric plate, an insulating cap having an upper wall, four side walls extending from the upper wall, and a pair of support members arranged to support the diaphragm at the inside of the two of the four side walls which are opposite each other, a plate shaped substrate having a first electrode section and a second electrode section, the diaphragm being located in the insulating cap, two of the four side edges of the diaphragm opposing each other being fixed to the pair of support members, a gap between the other two of the four side edges of the diaphragm and the cap being sealed by elastic sealing material, an acoustic space being provided between the diaphragm and the upper wall of the insulating cap, an opening edge of the four side walls of the insulating cap being bonded to the substrate, the metal plate being electrically connected to the first electrode section and the electrode disposed on the front surface of the substantially rectangular piezoelectric plate being electrically connected to the second electrode section.
The above described piezoelectric acoustic component also overcomes the problems with conventional devices and methods, as described with respect to the preferred embodiments described above.
According to this piezoelectric acoustic component, the opening of the insulating case in which the diaphragm is contained is sealed by the lid. Further, the metal plate is electrically connected to the first electrode section, and the electrode disposed on the front surface of the substantially rectangular piezoelectric plate is electrically connected to the second electrode section. In addition, the piezoelectric acoustic component can be constructed to provide a surface mounting type by extending the first and second electrode sections to the back-side of the substrate.
In this piezoelectric acoustic component, at least one of the electrode sections for achieving external connections may be an electrode film which is provided on the surface of the insulating case and extends from the support member to the bottom surface of the insulating case. Alternatively, at least one of the electrode sections for external connecting may be a metal terminal which is fixed to the insulating case and extends from the support member to the bottom surface of the insulating case. In this case, the metal terminal may be fixed to the insulating case by bonding, crimping or inserting or other suitable method.
Preferably, the piezoelectric plate is disposed on the metal plate in such a way that respective longer edge sides of the piezoelectric plate and the metal plate are aligned to each other and a respective one of the two shorter edge sides of the piezoelectric plate and the metal plate are aligned with each other so that the piezoelectric plate is disposed on the metal plate leaning towards the one shorter edge side of the metal plate at which the metal plate is supported by the support member of the insulating case. An exposed area is provided around the other shorter edge side of the metal plate and the diaphragm is fixed to the support members of the insulating case so that the metal plate opposes the bottom wall of the insulating cap. Further, it is preferable that the exposed area is connected to the first electrode section and the electrode disposed on the front surface of the substantially rectangular piezoelectric plate is electrically connected to the second electrode section preferably via an electroconductive adhesive agent.
It is possible to fix the diaphragm to the support members of the insulating case so that the piezoelectric plate opposes the bottom wall of the insulating case. But, in this case, since the electrode disposed on the front surface of the piezoelectric plate does not expose to the upper side, it is difficult to connect the electrode disposed on the front surface of the piezoelectric plate to the second electrode section of the substrate.
On the other hand, when the diaphragm is fixed to the support members of the insulating case so that the metal plate opposes the bottom-wall part of the insulating case, the electrode disposed on the front surface of the piezoelectric plate exposes to the upper side, and thereby it is easy to connect the electrode disposed on the front surface of the piezoelectric plate to the second electrode section of the substrate. Further, since the exposed area of the metal plate is exposed around the other edge side of the diaphragm, it is easy to connect the metal plate to the first electrode section of the substrate.
It is noted that the process of connecting the metal plate to the first electrode section of the substrate and the process of fixing the diaphragm to the support members of the insulating case may be performed at the same time. That is, electroconductive adhesive agent may be used when fixing the end of the diaphragm at the side of the exposed area to the support members.
Preferably, the elastic sealing material includes an insulating material and the elastic sealing material is provided at all of the four side edges of the diaphragm. It is noted that since the metal plate and the electrode disposed on the front surface of the piezoelectric plate are located close to each other, they are likely to be short-circuited when connecting the electrode disposed on the front surface of the piezoelectric plate and the second electrode section by electroconductive glue. If the elastic sealing material is provided in the periphery of the metal plate beforehand, such short-circuit can be prevented. Further, by sealing all of the four side edges of the diaphragm, air leakage is prevented to thereby greatly improve the sound pressure characteristic.
For providing the elastic sealing material at all of the four side edges of the diaphragm, a method of attaching the diaphragm to the insulating cap only by the elastic sealing material (without using adhesive) and a method of providing the elastic sealing material over the two edge sides of the diaphragm which are fixed by adhesive can be used. The latter method is advantageous when it is impossible to prevent air leakage only by the adhesive.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.